Osmium-ammine (OA)/SO2 selectively contrasted RNA- and DNA-containing structures in thin sections from Lowicryl-embedded samples. No cell structures were stained after Epon embedding. RNAse and DNAse digestion experiments demonstrated that only RNA and DNA were stained in Lowicryl thin sections. Protease digestion did not modify the staining reaction. The very fine end-reaction produced a very high resolution of the stained structures. The staining reaction was not due to the presence of SO2 but to the low pH of the solution (ranging from 1.5-2.2). OA in glycine buffer, pH 1.5, selectively contrasted nucleic acids. Electrostatic bonds between nucleic acids and OA complex were probably involved in the staining reaction. Increasing the pH value of the staining medium resulted in loss of OA specificity for nucleic acids. The high electrolyte concentration of the staining medium hindered the staining reaction.